1. Field of the Invention
This invention relates to a method for improving abrasion resistance at the sliding portion of a metal-product such as a machine part by: changing the metallic structure of the surface of the sliding portion; providing the surface of the sliding portion with a predetermined roughness; and forming sumps for oil to provide an oil film which is seldom broken. In the inventive method, a shot-peening process is performed on the surface of the sliding portion.
2. Description of the Prior Art
Conventionally, in order to prevent abrasion of the surface of the sliding portion of a metal-product, the hardness of the material of the metal-product was increased with a heat process such as hardening, carburization and nitriding. Alternatively, roughness of the surface of the metal-product was increased by machine work such as cutting, grinding and polishing with a machine tool such as a lathe and fraise.
Moreover, an oil film was formed at the sliding portion of the metal-product by means of an oil feed process, such as an oil bath process, splash process, drop process, cycling process or spray process, for preventing abrasion at the sliding portion. In these oil feeding processes, a method for effectively forming the oil film on the surface of the sliding portion of the metal-product and a method for improving quality of a lubricating oil for preventing the oil film from being broken have been developed.
Further, concave portions were formed on the surface of the sliding portion of the metal-product by means of scraping, cross polishing and the like, to form oil sumps. In scraping the surface of the sliding portion of the metal-product is chipped by scraping such as by spring scraping, plane scraping and cant scraping. On the other hand, cross polishing is a polishing method by which the surface of the metal-product is polished by cross-honing at high delivery speed using a honing machine. Cross polishing is used for polishing the internal surface of a cylinder in an engine.
In the conventional method for preventing abrasion on the surface of the sliding portion of the metal-product, in order to obtain further effective abrasion resistance, some of the above methods were combined. For example, roughness of the surface of the sliding portion of a metal-product was increased, hardness of the surface was increased by means of a heat process and an oil film was formed at the sliding portion by means of several kinds of oil feeding process.
However, the following problems remain in the above conventional methods for preventing abrasion on the surface of the sliding portion of a metal-product.
(1) When the surface of the sliding portion of the metal-product is hardened by means of the heat process, a strain appears in the metal-product. This causes a problem in that it takes much time to correct the form of the metal product. In some cases, the resultant metal-product may be inferior, having a form which can not be corrected.
In a metal-product, when the surface of its metal base or of its sliding portion is hardened by means of the heat process such as hardening, carburization and nitriding, the metal-product is lowered in toughness to become brittle. This causes a problem in that pitching occurs. For example, if the metal-product is a gear, pitching occurs at the corner of the end of each gear-tooth, i.e., the sliding portion of the gear.
(2) The above mentioned oil feeding methods cause problems in that apparatus for feeding the oil are complicated resulting in high costs.
(3) In the method for forming the oil sumps on the surface of the sliding portion of the metal-product by means of scraping, when the surface of the metal-product is too hard, it is difficult to perform the scraping. Thus, the heat process for increasing the hardness of the surface of the metal-product can not be perfectly carried out. Therefore, the abrasion resistance can not be improved sufficiently. Accordingly, when the sliding portion of the metal-product is subjected to sliding for a long time, the oil sumps formed on the sliding portion tend to become abraded or deformed. This causes a problem in that the oil film becomes broken. Further, under some abrasion-conditions at the sliding portion, the sliding portion needs adjustment or the metal-product itself needs replacement.
Since a large number of oil sumps can not be formed by means of scraping, the oil film becomes broken when the metal-product is subjected to sliding for a long time. Thus, the lubricating oil must be replaced or supplied periodically.
(4) On the surface subjected to machine work such as cutting, grinding and polishing, the cross sectional shape of the rough portion formed has an indented pattern with acute angles. Then, when the surface is subjected to sliding while lubricating oil is supplied thereto, the lubricating oil flows, due to capillary phenomenon, to the acute angled tip end of each concave portion in the rough-surface by a bearing pressure generated by sliding. This causes a problem in that the film becomes broken. The same problem is caused when the surface is cross polished.